An extruder generally comprises a screw-like element rotably mounted within a bore or screw channel formed in a longitudinally extended extruder barrel. The screw-like element includes various thread configurations which may alternate with mixing projections of various shapes and sizes such that the rotation of the screw-like element conveys and mixes ingredients which are fed into the screw channel through an inlet port arranged at the upstream end of the extruder barrel. Typically, an extrusion port or die is mounted at the downstream most end of the extruder barrel whereby the ingredients which have been mixed and conveyed by the rotating screw-like element are forced through the die to provide an output product which comprises a thorough mixture of the ingredients fed through the inlet port of the extruder.
In one advantageous embodiment of an extruder, twin screws are arranged within side-by-side, communicating bores which have a figure eight cross-section. The screw threads and mixing projections of the twin screws intermesh with one another in a self-cleaning action to mix, convey and otherwise mechanically work the various ingredients fed into the inlet port. The dimensions of the communicating bores and elements of the twin screws, as well as the speed of rotation of the twin screws can be designed such that the operation of the extruder introduces high compressive and shearing forces to the ingredients being mixed and conveyed throughout certain sections of the extruder to raise the temperature and thereby heat treat the ingredients during their conveyance through the extruder.
The heat treatment operation of an extruder also depends upon the viscosity of the mixed ingredients being conveyed. Generally, the viscosity must be high enough so that the viscous resistance of the mixture to the shearing and compressive forces introduced by the screw-like element results in sufficient friction to raise the temperature of the mixed ingredients. An extruder may also include a supplemental heat source, such as an electric heater in each barrel section arranged to surround the screw channel, to introduce heat, as for example, by conduction into the ingredients. Such a supplemental heat source could be used when the viscosity of the mixture is not high enough to provide the friction required for heat treatment. The barrel of the extruder may also include tubing for the circulation of, e.g., hot or cold water for additional temperature control of the screw channel. Moreover, a thermostat control can be used to maintain preselected temperatures at certain longitudinal sections or zones of the extruder. The thermostat may, for example, activate the electric heater to raise the temperature when the temperature level of a zone falls below the preselected level and further activate a source of cooling water through the tubing when the temperature level in the zone rises above the preselected level.
Extruders, particularly twin screw extruders, have been used widely in the food industry to continuously mix ingredients to produce a raw dough and also to at least partially cook the ingredients, as they are mixed, to produce a partially or fully cooked food product. Typically, extruders are used to produce cereals, snacks, pet foods and other products which generally are expanded upon extrusion through a die and which have relatively low amounts of oil as a percentage of the total weight of the food product. It has been found, for example, that the presence of oils above certain levels causes processing problems such as surging and product characteristics such as oil separation, which are undesirable. More specifically, upon extrusion, separated oil may appear as a coating on the remaining dough mass or periodically discharge separately from the remaining dough mass. Oil separation may be caused or aggravated by high pressure areas within the extruder as, for example, at the extruder die and within high compressive force zones in a heat treatment section of the extruder. In such areas, the compressive forces literally squeeze the oil from the dough mass.
In particular, it has been found that the use of an extruder to at least partially bake a cookie dough, which typically comprises a formulation including flour, added water and high amounts of both oil and crystalline sugar, may result in oil separation and excessive dissolution of the crystalline sugar. Traditionally, a baked cookie exhibits a crumb structure having structural integrity (i.e. the product does not readily crumble apart). A crumb structure is a significant characteristic which affects the texture of a cookie product. Oil separation caused by the high pressure, high temperature and/or turbulent conditions generated by the extruder, as required to achieve thorough mixing and heat treatment of a product within the extruder, tends to preclude a crumb structure. Moreover, the presence of high amounts of both oil and crystalline sugar with water above a certain level promotes the dissolution of the sugar in the water to increase the relative amount of hydrophilic mass which in turn, tends to cause further oil separation. The dissolved sugar may also result in an excessively hard texture for the final product. For these reasons, extruders have not been heretofore successfully utilized to at least partially bake a cookie dough to produce a final product having a cookie-crumb-like structure with structural integrity.
Conventional cookie production involves forming a cookie dough which is cut into pieces followed by baking of the pieces. Low temperatures, typically at about room temperature, are used to form the dough in a batch process. The low temperature mixing generally avoids separation of oil from hydrophilic dough components. While baking temperatures in a conventional oven, such as a band oven, may promote oil separation, there is no mixing or pressing action performed at the baking temperatures, as occurs in an extruder, to squeeze the oil from the dough mass. Any oil separation which may occur in such ovens does not generally interfere with continuous operability of the cookie production process as it would in a continuous cooker/extruder process.